1. Field of Invention
The present invention relates to a method for manufacturing a LOCOS structure. More particularly, the present invention relates to a method for manufacturing a LOCOS structure that has a reduced bird's beak area.
2. Description of Related Art
In the manufacturing of semiconductors, in order to avoid short-circuiting of adjacent devices, isolating structures are often constructed to separate the devices. The most common method for device isolation, besides using shallow trench isolation, is the local oxidation isolation method or LOCOS for short.
FIGS. 1A through 1C are cross-sectional views showing the progression of manufacturing steps in forming a conventional LOCOS structure. First, as shown in FIG. 1A, a pad oxide layer 11 is formed over a substrate 10 in oxidation process by placing the substrate in a furnace and heated in an atmosphere of oxygen. Next, a low pressure vapor deposition (LPCVD) method is used to form a silicon nitride layer 12 over the pad oxide layer 11. Conventional photolithographic and etching processes are then used to pattern the silicon nitride layer 12 into a structure as shown in FIG 1B. Principally, the silicon nitride layer 12 and the silicon oxide layer 11 are used mainly as a mask for forming the LOCOS structure and defining the active regions. Subsequently, impurity ions are implanted into the substrate, and then a wet oxidation method is used to grow a layer of field oxide 13 over the substrate. Because water and oxygen molecules are difficult to penetrate through a silicon nitride barrier, no silicon dioxide layer is formed above the silicon nitride covered substrate. For other areas where there is no silicon nitride coverage, a surface layer of the substrate will be oxidized to form a silicon dioxide layer. However, since water and oxygen molecules is able to diffuse horizontally near the edge of the silicon nitride layer, a portion of the substrate beneath the nitride edge will be oxidized to a varying degrees. Therefore, a final structure known as a bird's beak 13 is formed as shown in FIG 1C.
In practice, the formation of the aforementioned bird's beak shaped field oxide layer will cause problems in subsequent processes. For example, when the thickness ratio of silicon nitride/pad oxide is reduced, the transition areas where the bird's beak encroaches upon will be lengthened. This problem will intensify because of the constant miniaturization of devices nowadays. FIG. 2 is a top view of FIG 1C. From the sketch, it can be seen that the bird's beak structure occupies a large portion at the side of the active device region. On the other hand, if the silicon nitride/pad oxide thickness ratio is increased instead, then tensile stress created by the silicon nitride layer on the oxide layer will be increased considerably. Consequently, the quality of the gate oxide layer produced by the process will be poor.
In light of the foregoing, there is a need to provide an improved method of forming a LOCOS structure.